This invention relates to contacting siliceous materials, in regions hotter than about 300.degree. F., with an acidic solution of hydrogen fluoride such as those described in U.S. Pat. No. 3,889,753 by E. A. Richardson. The disclosures of that patent are incorporated herein by reference. The present invention relates to a process for formulating and utilizing such an acidic solution in a way that increases the amount of silica dissolved and decreases the amount of corrosion caused by the solution in such a hot environment.